Expiration dates of chemicals, drugs, and food products, which spoil over the course of time and have limited shelf lives, are determined to assure healthy usage. However, storage conditions of these products extremely affect their shelf lives. Products stored in unfavorable conditions spoil in a shorter time.
Different indicators for determining the storage conditions of products with limited shelf life are known. For example, some known intelligent temperature indicators are based on enzymatic or polymerization reactions and/or diffusion. In addition, radio frequency identification-time-temperature indicators (RFID-TTIs) exist.
An enzymatic time-temperature indicator, which has been improved by using alkaline phosphatase, is described in WO 9205415. In this method, the solution is colorless before the enzymatic reaction, whereas it gets yellow after the reaction occurs. In the aforementioned method, the color change to yellow cannot be monitored easily, so large amounts of indicator solution need to be used. Also, a carrier is used. These two factors increase the cost of production.
Another example related to known techniques is explained in WO 2006015961. Here, a time-temperature indicator is improved by using immobilized enzymes. Usage of immobilized enzymes causes some disadvantages, which include difficulties in measurements of immobilized enzyme activity, extra costs for immobilization, and the possibility of a decrease in immobilized enzyme activity during a prolonged storage period.
A time-temperature indicator described in U.S. Pat. No. 3,977,945 is based on the principle of fatty acid production from lipids by lipase activity resulting in a color change as a result of a decrease in pH. In this system, enzymes are captured in a matrix. Enzymes in the matrix system have lower activity than free enzymes. To increase the activity of enzymes in the matrix, enzymes must be used in larger quantities. This causes an increase in indicator cost.
A time-temperature indicator based on an enzymatic reaction has been disclosed in U.S. Pat. No. 4,826,762. Enzymes and other materials are dissolved in an organic solvent, which has a constant melting point. A disadvantage of this method is an indicator that is ineffective for temperatures under the melting point of the solvent but high enough to cause spoilage. Hexadecane is used as a solvent in the given example. When the temperature of the indicator is elevated above 18.2° C., the solid organic solvent melts, enzymatic reaction occurs, and the color of the indicator changes. However, the organic solvent becomes solid when exposed to temperatures below 18.2° C. Thus, food products that spoil when stored below 18.2° C. will not be identified.
Another enzymatic time-temperature indicator based on color change is described in U.S. Pat. No. 2,671,028. In this system, color changes as a result of reaction. Although an increase in temperature leads to an increase in the rate of color change, this indicator is inactive for temperatures that are not suitable for the storage of certain food products.
Still another intelligent label approach is disclosed in Turkish Patent No. TR 2007 0666B.
In general, commonly known intelligent labels have a problem of uncontrollable rapid color change, and require a physical barrier to keep the reactive components separated. To cure the above-identified problems, the present invention is generally directed to intelligent labels in which the color change can be controlled, and which do not require a physical barrier for separation of the reactive components.